Wherever electronic circuitry is coupled to an external cable run, a risk of damage to the circuitry, due to the transmission of transient overvoltages by the cable run, may occur. Such overvoltages may be due to any one of several factors. For example, lightning, electrostatic discharge, or malfunction of equipment at a remote end of the cable may be responsible. Several techniques exist for isolating circuitry from potentially damaging surges. These include, inductive coupling as shown in FIG. 1, capacitive coupling, and opto-isolation as shown in FIG. 2. Isolation transformers are usually used to implement inductive coupling.
FIG. 1 is a schematic diagram of a typical arrangement whereby a cable including a pair of conductors 2 and 4 terminate on an isolation transformer 6 which is connected to a load 12. In the event of a common mode voltage surge, the potential on both conductors 2 and 4 typically varies in the same manner, thereby causing substantially zero net current to flow through the primary coil 8 of transformer 6. As a result, the common mode transient does not induce a fault current in secondary coil 10, and so isolation transformer 6 provides common mode protection to load 12.
However, in the event that a voltage transient affects one of conductors 2 and 4 substantially more than the other, then a surge current typically flows through primary coil 8 and induces a transient voltage across the output terminals of secondary coil 10. The surge current may damage the isolation device and may also damage the equipment that is intended to be protected. Consequently, while an isolation transformer provides a good measure of protection from common mode transients, it does not provide protection from differential mode surges.
One area where isolation transformers are used significantly is in the implementation of local area computer data networks (LANs). With reference to FIG. 3, a digital processing card in the form of a network interface card (NIC), or as it is often called a “LAN card,” couples to J8-45 plug 22 of LAN cable 16 by means of J8-45 socket 18. LAN card 14 includes an isolation transformer module 20 that couples socket 18 to a data processing chip 24, which in turn communicates with the central processor 26 of a workstation by means of PCI slot connectors 28. Those PCI slot connectors 28 are received into a PCI connector, 30 which is in turn mounted on a mainboard 32 of the workstation.
FIG. 4 is a schematic diagram of the prior art card of FIG. 3, wherein like indicia are used to refer to like components. The card receives and transmits data pulses by means of respective balanced twisted pair wires that are enclosed in LAN cable 16 and that terminate on the RX+ and RX−pins and TX+ and TX− pins of J8-45 socket 18.
Isolation module 20 includes a pair of transformers 20A and 20B which respectively provide isolation for processing chip 24 from transients on the transmit and receive twisted pairs. As previously explained, isolation transformers 20A and 20B usually provide considerable immunity from damaging common mode transients but not from differential mode transients. For example, in the event of a voltage transient occurring at the RX+ pin but not on the RX− pin, that transient most likely will be transmitted across the isolation transformer and may damage processing chip 24.
It is therefore desirable to provide a convenient means for addressing the problems posed by differential mode transients as discussed above.